Abstract: | H2S increases the thermal isomerization of butene-2 cis (Bc) to butene-1 (B1) and butene-2 trans (Bt) around 500°C. This effect is interpreted on the basis of a free radical mechanism in which buten-2-yl and thiyl free radicals are the main chain carriers. B1 formation is essentially explainedby the metathetical steps: whereas the free radical part of Bt formation results from the addition–elimination processes: . It is shown that the initiation step of pure Bc thermal reaction is essentially unimolecular: and that a new initiation step occurs in the presence of H2S: . The rate constant ratio has been evaluated: and the best values of k1 and k1', consistent with this work and with thermochemical data, are . From thermochemical data of the literature and an “intrinsic value” of E?3 ? 2 kcal/mol given by Benson, further values of rate constants may be proposed: is shown to be E4 ? 3.5 ± 2 kcal/mol, of the same order as the activation energy of the corresponding metathetical step. |